SAT Question 3d04de9c: Linear Equations In One Variable answer and explanation with Desmos solution

Choice A is correct. It's given that a principal used a total of $25$ blue flags and yellow flags. It's also given that of the $25$ flags used, $20$ flags were blue. Subtracting the number of blue flags used from the total number of flags used results in the number of yellow flags used. It follows that the number of yellow flags used is $25-20$, or $5$.